I. Field of the Invention
The present invention is concerned with a device for the control of the internal pressure in a drill pipe or tremie for the construction of footings and especially in the construction of compressed concrete footings.
II. Description of the Prior Art
For the construction of compressed concrete footings the sunken drill pipe or tremie is sealed tightly at its upper end by means of a lid. Then compressed air is blown into the space which exists between the fresh concrete and the lid with which the drill pipe or tremie is sealed off. This amount of compressed air, which is enclosed in the drill pipe or tremie, results in a compression of the concrete which will cause a smoothing of the concrete with the ground when the drill pipe or tremie is withdrawn by means of a crane cable. In addition, the amount of compressed air assists in the withdrawal of the drill pipe or tremie by means of a cranecable due to the pressure at the bottom of the lid. The utilization of such an amount of compressed air in the drill pipe or tremie is potentially dangerous insofar as the friction of the drill pipe or tremie can suddenly be very much reduced, depending on the present friction in the ground and also depending on the rush of the ground water. This results in a sudden great acceleration of the drill pipe or tremie due to the effect of the amount of compressed air on the lid bottom in the direction of the withdrawal and the drill pipe or tremie will shoot out of the ground.
Such a sudden acceleration of the drill pipe or tremie is a disadvantage. It will normally result in damaged equipment and the operating personnel is endangered.
As a remedy for this disadvantage devices are known which serve for the control of the air pressure which exists inside the drill pipe or tremie during its withdrawal, and which devices are equipped with a valve which is affected by the internal pressure of the drill pipe or tremie in the direction of closing, and which valve opens in case of an acceleration of the upward movement of the drill pipe or tremie.
In case of one such known version (DE-PS No. 656 837) two such valves are provided in the lid which seal the drill pipe or tremie, which valves function in cooperation with a stop which is attached in a fixed position in relation to the drill pipe or tremie and by means of which stop the valves are opened when the drill pipe or tremie has moved a distance, which was determined previously. When the valves are opened, the internal pressure which exists in the drill pipe or tremie is relieved and, thus, the acceleration of the drill pipe or tremie is prevented or at least limited. This known version does not guarantee a dependable control of the internal pressure which exists in the drill pipe or tremie since in case of an acceleration momentum of the drill pipe or tremie which is too strong, the stop will be torn off and, therefore, the safety relief valve will be closed again immediately due to the internal pressure, such that the internal pressure is practically not released.
Another version (DE-PS No. 1 189 021) is known which eliminates the above described disadvantage where also a valve is provided in the lid of the drill pipe or tremie, which valve is in the direction of closing subjected to the internal pressure of the drill pipe or tremie, and which valve is further subjected to a counterpressure, the effective power of which counterpressure is larger than the power of the internal pressure. A hoisting cable is connected to the stem of the valve, the power of which hoisting cable in the direction of closing of the valve is adjustable, and which power under normal operating conditions in combination with the internal pressure of the drill pipe or tremie is large enough to overcome the counterpressure. The valve stem is equipped with a valve hat of smaller diameter which serves to seal the inner space of the drill pipe or tremie in case of the lifting of the valve hat. The inner space of the drill pipe or tremie is connected with venting holes which are located in the valve housing for the purpose of relief of the internal pressure. In addition, the valve stem is equipped with a ring-shaped piston of larger diameter, which ring-shaped piston forms a counterpressure chamber with the valve housing and the counterpressure chamber is connected with the inner space of the drill pipe or tremie by means of a drilled hole, which drilled hole is located axially in the valve stem, and which drilled hole extends through the valve head.
Because of the larger surface of the ring-shaped piston in comparison to the smaller surface of the valve hat, the pressure of the counterpressure chamber which is directed in the opening direction of the valve is always larger than the internal pressure of the drill pipe or tremie which is directed to the opposite direction, and the necessary additional power which is necessary for the maintaining of the closed position of the valve is added by means of the pull of the hoisting cable in the closing direction. As long as this proportion of forces exists at the valve, the upward motion of the drill pipe or tremie can be accelerated by means of its internal pressure. One must only take care that the force of the hoisting cable which acts on the valve in combination with the internal pressure of the drill pipe or tremie is sufficient at any time to overcome the counterpressure; however, this stable condition changes if the external forces which act on the drill pipe or tremie are suddenly reduced, such as, for instance, the friction between the wall of the drill pipe or tremie and the external ground or the poured concrete. In this instance the drill pipe or tremie is accelerated in the upward direction to a greater extent than before and the pull of the hoisting cable becomes ineffective; however, as a result of this the counterpressure which acts on the valve is greater, such that the valve hat is lifted and the internal pressure of the drill pipe or tremie is reduced and, thus, the acceleration of the drill pipe or tremie is stopped.
If the pull of the hoisting cable is represented with "Ps" and the internal pressure with "Pi" and the counterpressure with "Pg," the valve is closed if the following condition has been met: EQU Pg-Pi.ltoreq.Ps&lt;Pg
The pulling force of the hoisting cable must be controlled within these limits at a predetermined velocity if the valve should not release any compressed air from the inner space of the drill pipe or tremie. If the pulling force of the hoisting cable is reduced although the withdrawal speed of the drilling pipe or tremie was not increased, the valve will blow off and the withdrawal procedure is interrupted.
This known device, which is equipped with a valve, which valve is subjected to a counterpressure, has several disadvantages which endanger the operational safety. The most significant disadvantage consists in the fact that in case of lifting of the valve hat by means of the counterpressure, the counterpressure chamber is instantaneously relieved of pressure through the axial drilled hole of the valve stem. The rough operating conditions, which cannot be avoided when work with drill pipes or tremies is done, result in a danger of contamination of the valve by contaminants or soils which could consist, for instance, of cement, and which contaminants enter the valve housing and adhere to the inner wall of the valve housing and obstruct and hinder the complete opening of the valve due to the instantaneous relief of the counterpressure chamber which occurs already when the valve hat is lifted. Over a longer period of operation these contaminants can accumulate in the valve to such an extent that an opening of the valve by means of the counterpressure chamber is practically impossible and, thus, the internal pressure which exists in the drill pipe or tremie cannot be relieved. This disadvantage occurs especially in case of the construction of tilted compressed concrete footings where additional auxiliary equipment for the withdrawal of the pipe is used, such as, for instance, hydraulic pressure cylinders which are attached to the drill pipe. Therefore, the known valve does not guarantee an unobjectionable operation and, therefore, the upward acceleration of the drill pipe or tremie cannot be controlled in the necessary manner. Another disadvantage of the known valve consists in the fact that the valve hat strikes hard against its seat at the valve housing when the valve is closed by the hoisting cable, depending on the velocity of the hoisting cable which can amount to more than 0.7 meters per second. Due to this very sudden and very hard striking of the valve hat against its seat area, damage to the valve hat itself or damage to the valve seat area can easily occur, and which make a dependable operation impossible. In addition, the hard striking results in an annoying noise. Furthermore, it is another disadvantage of the known device that it is possible to blow compressed air into the inner space of the drill pipe or tremie when the valve is closed and before the lid is tightly connected with the drill pipe or tremie. In this way the lid can be lifted from the drill pipe. This can result in damage to the lid or damage to the drill pipe and, on the other hand, it represents a danger for the operating personnel.